Adaptive passive acoustic attenuation systems involve the adjustment of adjustable tuners, such as adjustable quarter wavelength resonators or Helmholtz resonators in a sound system or adjustable vibration absorbers in a vibration system. The adaptive passive tuners are adjusted to minimize an acoustic disturbance detected by one or more error sensors within the acoustic plant. Adaptive passive systems are particularly effective for attenuating narrow band acoustic disturbances, such as tonal disturbances.
Most adaptive passive acoustic attenuation systems have been implemented in the laboratory. Implementing a practical adaptive passive acoustic attenuation system at industrial sites or in other commercial applications involves significant changes in adaptive passive control techniques to accommodate the rigorous demands of industrial and/or commercial applications. Practical applications for adaptive passive silencing techniques typically involve higher acoustic loads and less pristine environments than has previously been experienced in laboratory experiments.
The most common adaptation algorithm for adaptive passive systems involves full and/or partial parameter space scanning. In this technique, the parameter setting of the tuners is changed in increments from some starting value to some final value (e.g. increments from a fully open adjustable tuner to a fully closed adjustable tuner) and the acoustic disturbance is monitored using an error sensor at each increment. The parameter setting is determined quickly by monitoring the error signal. However, this single scan technique has some drawbacks. First, time-varying disturbances in the acoustic plant can skew the results of the parameter space scanned. Second, random background noise at or near the frequency of interest can distort the error signal. One way to reduce distortion due to random background noise is to average the error signals over time. However, such averaging creates a time lag so that the actual optimum parameter setting is slightly earlier in time than that determined by the error scan.
Most laboratory experiments involving adaptive passive systems use a single adaptive passive element (e.g. an adjustable tuner) to provide attenuation. In commercial or industrial applications, a single adjustable tuner is usually inadequate. It is normally necessary to provide multiple tuners in order to obtain sufficient attenuation levels. One adaptation technique for multiple tuner systems is to adapt a single tuner at a time, but in many applications adjusting a single tuner does not create an observable change in the sound level. If sound level changes are not observable, adaptation is impossible. Even if sound level changes are observable, single tuner adaptation techniques suffer from slow adaptation in systems using multiple tuners. On the other hand, adapting all tuners in the system synchronously (i.e. identical passive parameter value for all tuners) provides obvious changes in sound level, and maximizes adaptation speed. This technique has significant drawbacks in practical applications, however. First, the technique creates annoying disturbances during the adaptation process. The adaptation process in most adaptive passive systems scans the range of passive parameter settings to determine an optimum setting. Scanning moves the passive parameter setting away from the optimal value. Scanning all of the tuners in the system contemporaneously produces more acoustic disturbance than scanning a single tuner at a time. Thus, overall acoustic levels increase significantly while scanning. Another drawback of scanning all tuners in the system synchronously is that the system requires a higher electrical power output capacity. Each tuner requires a certain amount of electrical power to scan, and synchronous scanning of all of the tuners in the system multiplies the power capacity requirements for the system. Higher system power capacity requirements increase the cost of the system. Yet another drawback of scanning all tuners in the system synchronously is that it increases the possibility of reaching a non-optimal global solution.
While adaptive passive acoustic attenuation can be usefuil for both sound control and vibration control, one particularly useful application for adaptive passive acoustic attenuation at the present time appears to be sound attenuation of tonal disturbances propagating through a duct. At low frequencies, sound propagates through a duct as a series of plane waves. Above a critical "cut on" frequency, however, sound can propagate in the plane wave mode plus one or more higher order modes. Commercial air duct systems typically have a large enough cross-section to support sound propagation in one or more higher order modes in the frequency range of interest for attenuation. Most laboratory adaptive passive acoustic attenuation systems are implemented in a duct having a relatively small cross-section so that sound can propagate in the plane wave mode only. Thus, most laboratory systems are designed to detect acoustic energy propagation in the plane wave mode only. A practical way to detect the total combined acoustic energy propagation in the plane wave mode and in the higher order modes normally present in commercial and industrial air duct systems is desirable.
Another problem in implementing adaptive passive acoustic attenuation in commercial and industrial applications relates to the fact that the frequency of the undesirable acoustic disturbance needs to be determined in a practical manner. In most laboratory systems, the frequency of the disturbance is known or assumed. However, in commercial or industrial applications, the frequency of the undesirable disturbance can change or drift over time.
Another problem in industrial and commercial applications is that disturbance levels can change radically. Adaptation under such circumstances using current adaptation algorithms can yield questionable results.
Since industrial and commercial applications are not typically pristine like laboratory environments, it is important that the adjustable tuners remain operational in the non-pristine environment. Nonetheless, in non-pristine environments, adaptive tuners are susceptible to mechanical failure. While it is desirable to reduce mechanical failure, it is also desirable to provide adaptation techniques that account for mechanical failure.